“Legacy P” Poses Long-Term Challenge To Water Quality

Phosphorus is one of the most important components in the ongoing struggle to balance agricultural prosperity with water quality. When farmers fertilize their fields with this essential nutrient, plants use some, with precipitation carrying excess amounts into nearby bodies of water, fueling algal blooms that can kill fish and and endanger drinking water quality.

But phosphorus that lingers in the soil, sometimes for decades, is troublesome as well.

What scientists call “legacy phosphorus” — or “legacy P,” a common abbreviation — exists in a sort of nutrient limbo.

Phosphorus can easily bind to soil particles, a state in which plants don’t usually access it. The nutrient generally must be in water plants take up in order for them to use it. Moreover, as farmers spread more fertilizer, new quantities of phosphorus are made available to crops. But as land erodes or heavy rains hit fields, some of that older phosphorus will make its way into streams, rivers, lakes and ultimately groundwater. In coming decades, greater quantities of legacy phosphorus could get flushed into water sources due to climate change, which is projected to increase heavy precipitation events in the Midwest.